sustainable roof technology. · ing the cooling season. th e solar power system consists of a...
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There are many types of sustainable
roofs including white roofs, green
roofs, and roofs with solar photovoltaic
(PV) panels and solar hot water systems.
Th e performance of sustainable roof
technology often can be optimized if it is
integrated with a diff erent, complementary
sustainable roof technology.
An example would be that the effi ciency of
a PV system frequently improves when it
is placed above a cool roof. Because of their
lighter color, cool roofs refl ect sunlight (solar
refl ectance) and effi ciently emit thermal
radiation (thermal emittance). By cooling
the roof and lessening heat transfer into the
building, cool roofs reduce the cooling load
of the air-conditioning system. Th is leads
to both energy and fi nancial savings while
improving sustainability by minimizing
greenhouse gas emissions.
Solar panels and cool roofs are a natural
marriage of sustainable technologies.
One solar installation company that has
embraced this type of marriage is Advanced
Powering Services of Rancho Santa Mar-
garita, Calif., which installed a cool roof/so-
lar panel beta test site on the roof of a local
industrial building. Tim Scharf, chief oper-
ating offi cer, said that although it’s too soon
for the beta system to generate long-term
data, the cool roof does seem to increase the
energy output of the solar panels.
“Based on the system we are using, we ex-
pected the energy output to be 10.5 kilowatt
hours (kWh) and, in fact, it is 11.8 kWh,”
Scharf said. “We attribute this increase to
the fact that the cool roof maintains a lower,
more optimal roof temperature that benefi ts
the performance of the solar panels.”
Scharf pointed out that solar systems work
best at temperatures below 90 degrees Fahr-
enheit; if temperatures exceed 110 degrees
Fahrenheit, the solar power output can be
reduced by as much as 50 percent.
The importance of a cool roofAn energy effi cient, cool roofi ng system
signifi cantly reduces roof temperatures
during the summer while improving the per-
formance of the PV system. Another benefi t
of a cool roof is that it can substantially
enhance the performance of solar panels by
lowering a building’s interior temperature,
Cool Roofs and Photovoltaics: An Unlikely PairMichael Magallanes
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anagement Journal
July/August 2011
thus reducing the power load needed for
air-conditioning. Other benefi ts include cut-
ting energy costs by keeping attics and ducts
cooler; improving occupant comfort; cutting
maintenance costs; increasing the lifecycle
of the roof; and reducing urban heat islands
along with associated smog.
Solar refl ectance and thermal emittance
are the two measures used to determine
the “coolness” of a roof. Solar Refl ectance
Index of a roofi ng product is a method for
determining the radiative properties of
roofi ng materials. SRI is defi ned by ASTM
Standard E1980-01 Standard Practice for
Calculating Solar Refl ectance Index of Hori-
zontal and Low-Sloped Opaque Surfaces.
Th e U.S. Environmental Protection Agency
summarizes SRI as “the relative steady-state
surface temperature with respect to the
standard white (SRI=100) and standard
black (SRI=0) under the standard solar and
ambient conditions.”
White refl ective coatings contain transparent
polymeric materials (such as acrylic) and
white pigment (such as titanium dioxide or
rutile) to make them opaque and refl ective.
Th ese coatings typically refl ect 70 percent to
80 percent of the sun’s energy. Despite the
white appearance, these pigments absorb
the 5 percent of the sun’s energy that falls in
the ultraviolet spectrum. Th us, the pigments
help protect the polymer material and the
substrate underneath from ultraviolet dam-
age. As long as the coating is white or light-
colored, the roof will have high refl ectance
and emittance levels.
Many factors influence actual benefitsAlthough the actual benefi ts of a cool roof
on a particular building are aff ected by
many factors—including building type,
load, season and climate zone—cool roofs
can signifi cantly reduce summer energy
usage. A reasonable annual energy savings
expectation for a typical low-rise retail or
other commercial building is 10 percent to
30 percent of the electricity usage for air
conditioning.
It has been found that a refl ective roof
coating can lower interior temperature of a
commercial or industrial building by eight
to 12 degrees Fahrenheit during the hottest
four hours of a summer day. Not only does
the lower interior temperature help reduce
energy expenditures, but it also improves
worker productivity, especially in non-
air-conditioned space, by creating a more
comfortable work environment.
A 2001 study by the Lawrence Berkeley
National Laboratory in cooperation with
the EPA underscores the value of cool roofs
for reducing energy usage and related costs.
Th e study focused on a 100,000-square-foot
building owned by a major retailer in Austin,
Texas, built with a traditional roofi ng system
featuring an exposed black rubber EPDM
membrane.
After 13 years of operation, the black roof
was replaced with a white thermoplastic
roofi ng system. Direct benefi ts of the new
roof included daily savings from reduced de-
mand for operation of the air-conditioning
system as well as present value of future
savings. Th ese savings were coupled with
benefi ts from energy conservation programs
sponsored by the local utility as well as state
and federal programs.
Specifi c benefi ts from the cool roof based on
2001 factors included the average summer
temperature of the cool roof compared to
the black roof was 42 percent less, drop-
ping from 168 degrees Fahrenheit to 126
degrees Fahrenheit. Th is increased the useful
life of the roof and reduced summer air-
conditioning usage by 14 percent resulting in
average monthly savings of US$490.
California’s Title 24 cool roof requirementSome of the factors that have led to a
growing number of cool roof installations
in California include the U.S. Department
of Energy’s ENERGY STAR® program and
the California Code of Regulations, Title 24,
also known as the California Building Stan-
dards Code, which prescribes cool roofs be
employed whenever low-slope commercial
roofs are constructed or replaced.
Bill Conley, CFM, CFMJ, LEED AP, IFMA
Fellow, an Aliso Viejo, Calif.-based facility
management and sustainability consultant,
is a proponent of marrying cool roofs
with solar. As a facility manager, his goal
is to achieve the best possible operating
confi guration that saves money and energy,
optimizes sustainability and ensures the
longest possible usage of a building.
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According to Conley, a long-time advocate
of sustainability, a cool roof combined with
solar can improve performance and econom-
ics of a PV system while also being an
important factor for California commercial
property owners complying with Title 24.
Echoing Conley, Ian McLaughlin with
Lineside Electric, a San Juan Capistrano,
Calif., installer of solar systems, stated, “A
cool roof as an integral component of a solar
installation enhances refl ectivity and other
benefi ts such as cooling a building’s interior
that can signifi cantly improve the operation
and effi ciency of the PV system.”
Willard Young with Solyndra Inc., a Fre-
mont, Calif., producer of innovative photo-
voltaics for commercial rooftop installations,
reported that based on his company’s data,
the output of its solar power system—which
consists of cylindrical collectors instead of
fl at panels—when combined with a cool
roof increases up to 20 percent due to the
improved collection of refl ected and diff use
light.
Solyndra is a major proponent of marrying
solar power with a cool roof. Th ree compa-
nies that combine cool roofs with Solyndra’s
system are Public Service Electric and Gas
Company, Norkus Foodtown and LPS
Industries, all based in New Jersey. Six of
Norkus’ grocery stores have white, refl ective
roofs that signifi cantly increase the energy
power generation and reduce cooling costs
by as much as 20 percent. LPS Industries,
which manufactures fl exible packaging that
protects items such as food, medical devices
and electronic products, has experienced
a similar level of energy savings. PSE&G
recently installed a 0.9-megawatt solar
system/cool roof combination on its Central
Division headquarters in Somerset, N.J.,
which should produce enough electricity to
power 155 average-sized homes.
The EPA National Computer CenterTh e EPA’s 101,000-square-foot National
Computer Center in the Research Triangle
of Raleigh-Durham, N.C., is one of the
more prominent buildings that combines
a cool roof with solar panels. Th e center,
which achieved a LEED-NC (Leadership
in Energy and Environmental Design -
New Construction) Silver rating in early
2005, handles a massive quantity of data
processing equipment. To achieve the Silver
rating and reduce the building’s substantial
power load, 15,000 square feet of the roof is
covered with solar panels on top of a highly
refl ective, ENERGY STAR compliant,
white membrane (reinforced thermoplastic
polyolefi n) that reduces unwanted heat dur-
ing the cooling season.
Th e solar power system consists of a
94-kWh (peak) PV array made up of
2,185 individual tiles. Each tile is a stacked
composite made of a layer of rigid polysty-
rene foam insulation board, a wiring chase
and airspace, and a PV module. Each tile
interlocks with adjacent tiles and rests on
top of the membrane-covered roof deck with
no mechanical penetrations. Th e output of
the PV array off sets approximately 5 percent
of the building’s electricity consumption,
which is estimated to be approximately twice
that of a conventional offi ce building of
equivalent size due to the large demand from
data-processing equipment.
Greg Eades, energy manager for the EPA
Research Triangle Park campus, said that
while he has no specifi c data on the aff ect
the cool roof has on the performance of the
PV system, it does appear that the roof is
benefi cial. Specifi cally, he pointed out the
energy output of the system has increased
over the past three years from 85,000 kWh
to 105,000 kWh while the “insolation”
factor—a measure of solar radiation energy
received on a given surface area in a given
time—has decreased.
“If you are considering a new roof, you should
seriously consider installing a cool roof and if
possible, a solar system on that roof all at the
same time.”
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July/August 2011
“One would expect that if the solar output is
increasing, the insolation also is increasing,”
Eades explained. “However, for this system,
the opposite is happening. Th e energy has
increased while the insolation has decreased,
which is counterintuitive. Although we don’t
have the data to prove it, one could surmise
the reason for the inverse relationship is the
presence of the cool roof. If that is the case,
the cool roof is defi nitely benefi cial.”
To enhance the EPA building’s energy effi -
ciency, a digitally controlled, fully-automated
building automation system (BAS) monitors
and controls various aspects of the building,
including temperature, pressure, humidity,
electrical systems, computer room cooling
units, cooling and heating equipment,
maintenance indicators, lighting and secu-
rity. Electronic sensors placed throughout
the facility communicate to the BAS when
temperature, humidity, fresh air ventilation
rates and other environmental conditions
need to be adjusted, further increasing the
facility’s energy effi ciency. Th e EPA also
took advantage of numerous opportunities
to optimize the design for effi ciency in its
mechanical specifi cations.
The importance of roof conditionGetting down to basics, if a roof needs to
be replaced or patched after solar panels
has been deployed and operational, the lost
revenue to the owner can be substantial.
According to David Montross, president
of Montross Roofi ng—an Orange County,
Calif., fi rm that specializes in roofi ng, deck-
ing and construction services—a careful
inspection of the roof system before solar is
employed not only is necessary but also criti-
cal. If the roof shows signs of weathering or
wear and tear such as blistering and splitting,
then it’s probably best to re-roof with a cool
roof before installing solar panels. For the
optimum performance of the investment, a
roof should be able to last at least 20 years
from the time of PV installation.
“If you are considering a new roof, you
should seriously consider installing a cool
roof and if possible, a solar system on
that roof all at the same time,” Montross
explained. “A cool roof reduces building
cooling requirements by lowering the
temperature of the roof and the building
underneath. Th is means cooling equipment
savings and, in many cases, the ability to run
less air conditioning or purchase smaller
air conditioning units. A cool roof also will
increase the life of a roof. By lowering the
roof temperature, roofi ng products may last
longer due to less thermal stress over time.”
FMJ
Michael Magallanes is vice president of Coat‘N’Cool (www.coatncool.com). Coat‘N’Cool is a Yorba Linda, Calif., company that has created a proprietary cool roof product that is an easy-to-apply architectural
coating specially engineered to reflect sun light in the UV, IR and Visible spectrum. Magallanes may be reached at [email protected].